In an image forming apparatus that forms an image electrophotographically, a developing roller is provided opposite to a photosensitive drum. An electrostatic latent image is formed on a surface of the photosensitive drum. The developing housing accommodates toner. The developing roller is rotatably held in the developing housing. A developing bias is applied to the developing roller. When the photosensitive drum is rotated and an electrostatic latent image on the surface of a photosensitive drum opposes the developing roller, toner is supplied from the developing roller to the electrostatic latent image by means of a potential difference between the photosensitive drum and the developing roller. According thereto, the electrostatic latent image is developed into a toner image, and the toner image is formed on the photosensitive drum. The toner image is transferred to a sheet directly or by way of an intermediate transfer belt from the surface of the photosensitive drum.
However, when the toner image is formed in the number of times, the toner held on the developing roller may be deteriorated. For example, quantity of electric charge of the toner may be decreased. Thus, the quantity of toner transferred from the developing roller to the surface of the corresponding photosensitive drum may eventually vary, and density of an image (i.e., a toner image) to be formed on the sheet differs from an appropriate density. For this reason, the image forming apparatus makes a correction to a value of the developing bias at appropriate timing in such a way that a toner image of appropriate density is formed.
By correcting the value of the developing bias, it is possible to constantly maintain the density of the image formed on the sheet to a certain degree before the toner becomes considerably deteriorated. However, when the toner becomes significantly deteriorated, it is no longer possible to form the image in an appropriate density on the sheet by only adjusting the developing bias. In addition, the toner may adhere to a white area (an area outside a print area where the image is to be formed) of the sheet, and a phenomenon of so-called print fog may occur. In addition, when the image is developed while toner is significantly deteriorated, the toner may leak from the developing housing. Therefore, when the toner becomes considerably deteriorated, the developing housing or a developing device including the developing housing is considered to have reached the end of its life, and it becomes necessary to replace the developing device with a new one.
There has been proposed a known image forming apparatus which, when a remaining quantity of the toner in the developing housing becomes equal to or less than a predetermined quantity, determines that the toner has become empty, prohibits subsequent formation of images on recording mediums, and notifies a user to replace the developing device.